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Keywords:

  • noctilucent clouds;
  • mesospheric tides;
  • polar mesospheric clouds

As the Sun dips below the horizon, the last rays of light can glint off crystals of ice high in the atmosphere, lighting up the sky with an electric blue glow. Known as noctilucent clouds, these collections of ice crystals occur most often at high latitudes, but the long polar days make them difficult to see. With the eye, noctilucent clouds can best be seen at night between 50° and 60° latitude in both hemispheres.